This invention relates to syringes and more particularly to such devices which are capable of being operated by one hand and which are used to withdraw liquid, such as blood samples, from animals.
Conventional syringes comprise a cylinder having an apertured needle at one end and a piston in the cylinder, with the piston being connected to a rod extending from the other end. In order to withdraw liquids, it is necessary to hold the cylinder in one hand and pull the rod with the other. It would be desirable to provide a syringe which is capable of withdrawing fluids upon compression of the piston, which would allow for one handed operation with the use of finger tabs on the cylinder.
The U.S. Pat. No. 4,813,433 discloses a syringe having a pair of connected chambers, with a piston and rod in one of the chambers. Forward movement of the piston in one chamber causes reduced pressure in the other chamber, the latter being connected to a hollow needle. This concept requires a particular seal for the shaft, which may not be particularly reliable, or may expose the liquid compartment to non-sterile conditions. German patent No. 621145 discloses the use of a reverse action gear on the syringe plunger whereby a pushing action on the plunger is translated into a pulling action on the piston.